The high hot hardness of a ceramic material allows the ceramic material for cutting tools to be an appropriate choice for machining a workpiece which has high hardness, high tensile strength and low thermal expansion at elevated temperature, and it is particularly suitable for a self-hardening material such as nickel- or cobalt-based material which is often used for the heat resistant super alloy (HRSA).
Many nitride silicon materials for cutting tools are fabricated using aluminum oxide Al2O3 as a sintering additive. Because aluminum and oxygen can replace the silicon and nitrogen in silicon nitride crystal structure respectively, these can form so-called “sialon” ceramic. The sialon ceramic consists of silicon, aluminum, oxygen and nitrogen (Si—Al—O—N) and can sometimes be additionally stabilized by cation (Me), in which Men+ may refer to a material having proper ion radius (r<1.0 Å) with a plurality of rare earth metals, and may be selected from lanthanide such as Y, Yb, Dy, Lu, Li, Ca, Mg, Sc, for example.
Referring to “Progress in SiAlON ceramics” by Izhevskyi V A, Genova L A, Bressiani J C, and Aldinger F (J. Eur. Ceram. Soc. 20, 2275-2295(2000)), among a large number of sialon phases detected and characterized, the main phase for use in cutting tools remains in α-sialon phase, i.e., in RxSi12-(m+n)Al(m+n)OnN(16-n) (1.0<m<2.7; n<1.2) state, in which R is one of the above-mentioned metals or the lantanoide having smaller ion radius than 1.0 Å, and β-sialon is Si6-zAlzOzN(8-z) (0<z<4.2).
During sintering, the materials such as silicon nitride, alumina, AlN or a number of materials such as 12H, 21R used as a sialon polyphase (or polytype) composition temporarily form a molten material with lanthanoid or metal, or with lantanoid oxide. From this molten material, α- and β-sialon phases (if Y is used as metal ion R described above), and possibly, different phases such as YAG, melilite, B-phase, 12H and so on are crystallized. After sintering, the intergranular phase among crystalline particles remains. The amount of the produced intergranular phase is influenced by the composition of the materials and sintering conditions used.
The metal ion can not only act to stabilize the α-sialon phase, but also act as a catalyst to form sialon crystals during sintering. It usually aids forming of elongated sialon particles in 3-phase, but elongated α-sialon particles are also produced. (Refer to “Nucleation and Growth of the Elongated α-SiAlON” by Fang-Fang X, Shu-Lin W, Nordberg L O, Ekstrom T (J. Eur. Ceram. Soc. 17(13) 1631-1638 (1997)).
U.S. Pat. No. 7,629,281 proposes a ceramic material comprising β-sialon, 12H, and intergranular amorphous or partly crystalline phase and yittrium.